The present invention relates to a.c. power measurement in general, and more specifically, to a device for measuring power by sensing a.c. currents accurately over a wide temperature range and wide dynamic range of applied currents.
Power measurement technology has developed three main approaches to measuring current: current transformers, shunts and Hall effect and like devices. Current modern electronic electric utility power meters must handle a very wide dynamic range from 200 Amperes down to Milliamperes and each approach has its limitations. Conventional current transformers exhibit a very limited dynamic range, since they saturate at high currents and they lose sensitivity because of limited initial permeability. Current transformers also tend to saturate with small d.c. current flow caused by half-wave rectified loads, and they exhibit non-linear response because of the magnetizing current which causes amplitude and phase shift errors of the measured currents. Since instantaneous power is the product of instantaneous voltage and instantaneous current, any phase shifts can cause errors.
Current transformers generally use a large, high quality toroid transformer for the highest accuracy. To reduce cost and size a shunt is often used.
Shunts, i.e., resistive shunt measuring devices, are desirable because of their low cost compared to current transformers but exhibit several limitations. Although measured voltage drop in a shunt is proportional to current, heating is proportional to the square of the current. Hence, shunts tend to waste power and can overheat to the point of destruction in a wide dynamic range environment. A shunt measuring circuit must be at the same potential as the shunt. This restriction makes it awkward to measure two simultaneous currents, as for example in 120/240 volt circuits where each is at a different potential.
The inability of shunts to accurately track current over a wide temperature range can be at least partially attributed to various materials used in making the shunts. Accuracies on the order of a few parts per million per .degree.C. are required, but are not feasible as the resistive material must also be able to withstand 7,000 Amperes short circuit current without change of accuracy. One material used in shunts is Manganin. Its characteristics allow very accurate and uniform current tracking with respect to the change in temperature. However, it is very difficult to work into the elements of a transformer having a shunt. When the solid metal is shaped into a desired geometry, much of the desired current tracking capabilities are lost for unknown reasons. Another material having uniform resistivity with respect to temperature change is Coopernal. However, Coopernal, too, cannot be worked into desired shapes such as a complex bridge piece forming a shunt without losing its desired uniform resistivity and temperature stability.
Electronic sensors, such as Hall effect devices, exhibit marked temperature sensitivity and provide limited long-term stability. This is a limitation for many applications.
Therefore, what is needed is a current measuring device with improved current tracking accuracy between a shunt portion and main load portion over a wide dynamic range and wide temperature fluctuations.